More docs

compiler/rustc_middle/src/ty/generics.rs

@@ -122,6 +122,10 @@ pub struct Generics {
 }
 
 impl<'tcx> Generics {
+    /// Looks through the generics and all parents to find the index of the
+    /// given param def-id. This is in comparison to the `param_def_id_to_index`
+    /// struct member, which only stores information about this item's own
+    /// generics.
     pub fn param_def_id_to_index(&self, tcx: TyCtxt<'tcx>, def_id: DefId) -> Option<u32> {
         if let Some(idx) = self.param_def_id_to_index.get(&def_id) {
             Some(*idx)